Missing top-level class documentation comment.

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This cop checks for missing top-level documentation of
classes and modules. Classes with no body are exempt from the
check and so are namespace modules - modules that have nothing in
their bodies except classes, other modules, or constant definitions.

The documentation requirement is annulled if the class or module has
a "#:nodoc:" comment next to it. Likewise, "#:nodoc: all" does the
same for all its children.